It IS a caterpillar. There are some moth species in which the caterpillars feed on aquatic plants, and so are able to live underwater.
The species found in Singapore are not really well known, but here are some links to websites discussing aquatic moths in general:
http://www.ecospark.ca/changingcurrents/aquaticmoth
http://watermonitoring.uwex.edu/wav/...pidoptera.html
http://davehubbleecology.blogspot.sg...tic-moths.html
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